1. Field of the Invention
The present invention relates to a speed-changing transmission hub for a bicycle, particularly to a speed-changing transmission hub for a bicycle, which does not require switching the torque input and has a group of one-way clutches for controlling the fixing of the sun gear group.
2. Description of Related Art
A conventional speed-changing transmission hub uses a planetary gear train comprising a ring gear, planet gears and sun gears. Torque is inputted to the planetary gear train either via the ring gear or the carrier, as controlled by a clutch, which engages either with the ring gear or with the carrier, thus controlling the path of torque within the planetary gear train and the degrees of freedom of the sun gears for various speeds. Depending on the characteristics of the planetary gear train, the tendency of the rotation of the sun gears is connected to which of the gears of the planetary gear train is active. Different torque inputs (by the ring gear or by the carrier) lead to different tendency of the rotation of the sun gears. If the carrier is driving, the sun gears rotate the same way as the carrier. If the ring gear is driving, the sun gears and the ring gear rotate the opposite way. U.S. Pat. No. 5,078,664 discloses a device for controlling the degree of freedom of the sun gears, wherein fixing each sun gear is controlled by two one-way clutches working in different directions. This arrangement, however, is complicated.
In order to resolve this problem, U.S. Pat. No. 5,273,500 discloses a two-stage planetary gear train with a common carrier. Therein, each sun gear is provided with a single one-way clutch, which is sufficient to control its rotation. However, since more than one sun gear and ring gear are needed, the system becomes weighty and complicated.